1. Field of the Invention
The present invention relates to a method for a servo control, by which the position in the width direction of the magnetic head is controlled based on the servo signal written on a magnetic tape.
The present invention, furthermore, relates to a record-reproduce equipment for magnetic tape, a servo writer equipped in the record-reproduce equipment, and a magnetic tape used in the record-reproduce equipment for a magnetic tape.
2. Related Art
In recent years, a high-density recording technique applied to a magnetic tape has been developed rapidly. In the case of a magnetic tape to be used for a backup of a data of computer, for example, it is well known that a magnetic tape having about 100 G bytes recording capacity.
For recording such large size data, data tracks of several hundreds are formed in a line along the width direction of a magnetic tape. Thereby, not only the width of the data track itself but also the gap between the adjoining data track become extremely narrow.
The record-reproduce equipment, which is used for recording/reproducing the data on/from the magnetic tape, equips with a magnetic head including a plurality of record-reproduce devices.
In the record-reproduce equipment of the magnetic tape adapted for a high-density recording, generally, the numbers of the read-write devices are not same as the numbers of the data tracks.
Thus, the recording or reproducing of the data is performed by changing the data track in due order.
To be more precise, when the recording or reproducing of the data by the magnetic head is performed from one end to another end of the data track, the position in the width direction of the magnetic head is shifted to just above the adjoining data track. Then, the recording or reproducing of the data by the magnetic head is performed from another end to one end. By repeating this motion of the magnetic head for several times, the recording/reproducing of the data on/from the magnetic tape is achieved.
In the record-reproduce equipment of the magnetic tape, therefore, the position of the magnetic head should be accurately controlled so that the magnetic head is positioned just above the data track for achieving the recording or reproducing of the data.
The magnetic tape, generally, is a thin film. Thus, the subtle winding in the width direction of the magnetic tape may be arisen when the magnetic tape is fed to the longitudinal direction by the record-reproduce equipment.
If such winding of the magnetic tape is arisen, the relative location between the data track and the recording-reproducing device may go wrong. In other words, the position of the record-reproduce device may not agree with the data track because the position of the magnetic head is displaces in the width direction with respect to the magnetic tape.
For alleviating this problem caused from the miss-location, a servo-control method, in which the position in the width directions of the magnetic head is controlled based on the servo signal written on the magnetic tape, has been known.
In this servo-control method, the writing of the servo signal to the servo track of the magnetic tape is performed by the servo writer. The positioning of the magnetic head is performed based on the servo signal, which is readout by the record-reproduce equipment.
As another examples of servo-control methods, a timing based servo method and an amplitude servo method are also well known. In the timing based servo method, the positioning is performed based on the servo timing of the servo pattern. In the amplitude servo method, on the other hand, the positioning is preformed based on the width of the servo pattern.
In the case of the timing based servo method, servo signal is generally written on a plurality of servo tracks, which are arranged in the width direction of the magnetic tape. The servo signal is a signal consists of a group of servo pattern units, in which each servo pattern unit is arranged at the predetermined interval along the longitudinal direction of the magnetic tape.
The servo pattern unit is also consisting of a plurality of nonparallel servo patterns. This nonparallel servo pattern is pair of patterns, in which each pattern are located symmetrically with respect to the width direction axis of the magnetic tape in the condition that each pattern has a predetermined intersection angle with respect to the longitudinal direction axis of the magnetic tape.
The servo writer has a servo write head, which is used to write the servo signal on the magnetic tape.
The servo write head has a plurality of head gaps having a shape of unparallel servo pattern. The numbers of head gaps are same as the numbers of servo tracks of the magnetic tape.
In this servo writer, the writing of the servo signal on the magnetic tape is achieved using the magnetic flux, which is generated by supplying the writing current to the coil of the servo write head and is leaked from the head gap.
The writing current consists of a zero current and a pulse current having a positive or negative polarity. When the pulse current of positive or negative polarity is supplied at predetermined timing, each nonparallel servo pattern provided on the servo write head is magnetized.
Thereby, the signal, which has a plurality of the magnetized servo patterns in the longitudinal direction of the magnetic tape, is written on the magnetic tape.
In the record-reproduce equipment of the magnetic tape, incidentally, a servo signal readout device is provided on the magnetic head. The servo signal readout device is composed of MR devices (Magneto Resistive device). In this servo signal readout device, the readout of the servo signal is achieved based on the change of the electric resistance of the MR device. This change of the electric resistance is caused by the external magnetic field.
In the record-reproduce equipment of the magnetic tape, furthermore, the position of the servo signal readout device is changed when the recording or reproducing of the data on or form the magnetic tape is performed from one end to another end of the magnetic tape. In that occasion, the servo signal readout device is shifted with same shifting amount at every shifting.
As an example of the servo signal, as shown in FIG. 12, servo signals SS recorded on servo tracks ST is known. These servo tracks ST are provided in the width direction of the magnetic tape MT.
In each servo signal SS, two nonparallel servo patterns SP and SP are arranged with short interval (C) along the longitudinal direction of the magnetic tape MT. This unit of two nonparallel servo patterns is also arranged with long intervals (B).
In the record-reproduce equipment of the magnetic tape, generally, the positioning of the magnetic head is performed based on one servo signal SS if readout of the servo signal SS can accurately be achieved.
To be more precise, the displacement in the width direction of the servo signal readout device, a magnetic head, is detected based on the ratio between the interval A1 and interval B. Here, the interval A1 corresponds to the distance between each pattern of the nonparallel servo pattern, and the interval B corresponds to the distance between the parallel patterns of each servo pattern units, which are adjoining with large interval.
The position of the servo signal readout device is in proportion with the ratio between the interval A1 and interval B. The position of the servo signal readout device is computed based on Formula (1) when the length between the nonparallel servo patterns SP is narrow. On the contrary, the position of the servo signal read out device is computed based on Formula (2) when the length between the nonparallel servo patterns SP is wide.POS1=k(A1)/(B)  (1)POS2=k(A2)/(B)  (2)
Here, POS1 is the computed position of the servo signal readout device, wherein the readout is performed at the narrowest position of the nonparallel servo pattern. POS2 is a computed position of the servo signal readout device, wherein the readout is performed at the widest position of the nonparallel servo pattern. A1 is an interval between the patterns of the nonparallel servo pattern, wherein the readout is performed at the narrowest position of the nonparallel servo pattern.
A2 is an interval between the patterns of the nonparallel servo pattern, wherein the readout is performed at the widest position of the nonparallel servo pattern. B is an interval between the servo pattern units. K is a coefficient.
In such types of the record-reproduce equipment of the magnetic tape, however, when the expansion and the contraction of the magnetic tape or the change of the feeding speed of the magnetic tape is arisen while recording the servo signal on the magnetic tape using the servo writer, the detecting cycle of the long interval B of the servo signal SS may be changed.
In that occasion, as shown in FIG. 12, the detecting cycle of the interval B may be changed as “B+ΔB” or “B−ΔB”.
When the readout by the servo signal readout device is performed at the position where the interval of the nonparallel servo pattern is narrowest and if detected long interval is B+ΔB, the position of the servo signal readout device is computed from Formula (3).
On the other hand, when the readout by the servo signal readout device is performed at the position where the interval of the nonparallel servo pattern is widest and if detected long interval is B+ΔB, the position of the servo signal readout device is computed from Formula (4).
In that occasion, the fluctuation quantity of the computed position of the servo signal readout device is obtained from Formula (5) and Formula (6).POS1′=k(A1)/(B+ΔB)  (3)POS2′=k(A2)/(B+ΔB)  (4)ΔPOS1=|POS1′−POS1|=k(A1ΔB)/B(B+ΔB)  (5)ΔPOS2=|POS2′−POS2|=k(A2ΔB)/B(B+ΔB)  (6)ΔPOS1<ΔPOS2  (7)
Here, POS1′ is the computed position of the servo signal readout device, wherein the readout is performed at the narrowest position of the nonparallel servo pattern and the detected long interval is B+ΔB. POS2′ is a computed position of the servo signal readout device, wherein the readout is performed at the widest position of the nonparallel servo pattern and the detected long interval is B+ΔB. A1 is an interval between the patterns of the nonparallel servo pattern, wherein the readout is performed at the narrowest position of the nonparallel servo pattern. A2 is an interval between the patterns of the nonparallel servo pattern, wherein the readout is performed at the widest position of the nonparallel servo pattern. B is an interval between the servo pattern units. K is a coefficient.
As can be seen from Formula (3) or Formula (4), the computed position of the servo signal readout device is varied owing to the variation of the detecting cycle of the long interval B, when the writing position of the servo pattern SP is varied in the longitudinal directions. In other words, the computed position of the servo signal readout device is varied owing to the variation amount ΔB.
Thus, the record-reproduce equipment of the magnetic tape considers that the displacement in the width direction of the readout position is arisen, and controls the position of the magnetic head of the servo signal readout device in order to correct the displacement. This positioning of the magnetic head is performed even if the readout position in the width direction of the magnetic head is accurate.
As can be seen from Formula (7), which shows the magnitude relation between Formula (5) and Formula (6), the variation quantity of the computed position of the servo signal readout device, which is obtained when the readout is performed at the widest interval, becomes larger than the variation quantity of the computed position of the servo signal readout device, which is obtained when the readout is performed at the narrowest interval. Here, the readout performed at the widest interval means that the readout is performed at the position where the interval of the patterns of the nonparallel servo pattern is wide. The readout performed at the narrowest interval means that the readout is performed at the position where the interval of the patterns of the nonparallel servo pattern is narrow.
That is, the variation quantity (detecting error) in the width direction of the computed position becomes larger as the readout position in the width direction of the servo signal readout device approaches to the wide interval side of the patterns of the nonparallel servo pattern. Thereby, the positioning quantity of the servo signal readout device becomes larger.